Worm hob



June 14, 1932,

N. TRBOJEVICH 1 Filed-July 21, 1930 2 Sheets-Sheet l ATTORNEYS w M V mJune 14, 1932.

NI TRBOJEVICH wow aoB Filed July 21. 1930- 2 Sheets-Sheet 2 Az/YaJcz nbyg ATTORNEYS Patented June 14, 1932 NIIOLJ. 133303317103, 01 DETROIT,MICHIGAN WORM HOB .lppltcation filed July 21, 1930. flerlal No. 469,577.

The invention relates to a worm hob of the tangential feed type andresides in a novel construction of flutes and a novel method ofrelieving the cutting teeth of the said hob.

The principle on which the new hob operates is somewhat similar to theprinciple exhibited in my prior application for patent ta 5, Serial N 0.204,951 filed July 4, 1927.

he object of this invention is to provide In a hob of the specified typewhich although relieved in all its active cutting edges will not loseeither its pitch diameter or the thickness of teeth after repeatedsharpenings.

Another object is to construct a hob that 13 will ream out or burnishthe teeth of the gear to be cut in the same operation in which they aregenerated.

Another object is to. provide a hob ground in the thread in which thegrindin operation extends only to certain burnisfiing areas found inthis hob and not to the roughing teeth, thereby reducing the cost ofgrindin Still another object is to roduce in a b5) a series of roughingteet having an increased chip room and a series of finishing teethhaving a reduced chip room but an increased strength and ri idity.Another object is to construct a hob liaving roughing and finishingportions of thread in which the roughing teeth merge into the finishingteeth imperceptibly, that is, without causing the finishing teeth tobite oil a chip thicker than a predetermined small dimension, usuallyless than one thousandth of an inch thick and further to preserve thisnovel quality of the hob after sharpenings.

In the drawings:

Fig. 1 is a side-elevation of myimproved hob.

Fig. 2 is the same elevation diagrammatically represented.

Fig. 3 is a cross section of Fig. 2 in the plane 3-3.

Fig. 4 is another section in the plane 4-4.

Fig. 5 is still another section in the plane 5-5.

Fig. 6 is a diagram explanatory of the relieved hob tooth.

Fig. 7 is the plane development of the pitch cylinder of the hob shownin Fig. 2.

Fig. 8 is an enlarged and exaggerated detail of Fig. 7.

Fig. 9 is a hob tooth perspective explaining the meaning of thetermsnormal pressure angle and side clearance.

The'new hob (see Figs. 1 and 2) is formed from a steel worm, the threadsof which are longitudinally intersected by means of a pluw rality offlutes 11, thereby forming a plurality of cutting teeth 16. The flutes11 are of a novel construction in that they consist of two superposedsystems of flutes, one of a shorter and one of a longer helical leadthus producing compound flutes which are wider at the roughing end 12 ofthe hob and narrower at the finishing end 13. The hob is provided with aplurality of irregularly bounded true helicoidal surfaces 14 prefer- 79ably accurately ground over after hardening, said surfaces beingdistributed adjacent to the front or cutting faces 15 of the teeth 16 ingenerally increasing areas as approaching towards the rear or large end13 of the ho The presence of these non-relieved helicoidal surfaces isessential to the proper functioning of the new hob and serves twopurposes, first the said areas catch and removed the minute particlesand surpluses of metal 39 left over b the preceding roughing andfinishing teet thus producing smooth and accurate tooth surfaces andsecond, they form the reserve or the life of the hob so that it mawithstand a considerable number (about fi teen to twenty-five)resharpenings without losing the size or form. Thus, when this hob isresharpened, the face 15 of the lines 11 is ground over and the areas 14will thereby be diminished. When all such areas are eventually groundoil the hob will be spent and will no longer produce the exact work forwhich it was intended; however, as long as the said areas remain in atleast one tooth, the hob will cut true to size. i

The front end of the hob (Fig. 2) is a truncated cone 18 approximatelycomprising in length onehalf of the total length of the hob. Thecompound flutes 11 are denoted with-the numeral 19 for the flute of thelonger lead ning from the said helix 21 will be re ieved. The

flutes of the above described divergent orcompound type.

The method of backing off the new hob will now be described. As shown inthe pitch plane development, Fig. 7, we first mill the uired number offlutes along the helix 19 (t e helix of the longer lead) thus cutting sbounded by the helixes 19 and 20. The ho teeth 16 are now backed oifusing the parallel helix 21 as the beginof the back off in such a mannerthat the portions of the teeth lying between helixes 20 and 21 willremain cylindrical or non-relieved whilethe portions lyin outwardlydistance b of the helix 21 from the helix 20 is proportional to thedistances b, Fig. 5.

After havin the teeth 16, thus, partially completed in t at each toothwill now be of a hexa nal contour B B C C D D? (Fig. 7), comprising anon-relieved portion B B E E, in the form of a parallelogram in front ofit and a trapezium E E D in the rear of it, we intersect the entireseries of cuttin teeth by means of the helical flute 15 of a s orterlead, said flute forming an angle G with the flute 21. The effect 'ofthis last operation will be to make each tooth in the hob different fromany other tooth; howevenall teeth takon together will form an organicseries exhibiting gradual variations, which is a matter of importance inthis theory.

The introduction of the second helix 15 produces two important results;first, all teeth to the left of the point A, which is the intersectionof the hclixes 15 and 21 respec- .time the point A reaches, thus, p p ofthe hob, the hob will be spent, but not'unt1l""' tively, are of an everdecreasing thickness C C as counted from the said point A, while theteeth to the right of the lpoint. A are all of a uniform thickness but apossess cylindrical lands H H of an ever increasing width as countedfrom the point A. Thus, in one single operation we ave produced first, aserles of roughin of an ever increasing width so that each toot willtake its proportional share of the work, second, a finishing tooth atthe point A and third, a series of burnishing teeth having burnishingsurfaces arranged according to ever increasing areas.

Still further, these conditions will not change after resharpening. Letus sharpen the hob along the helix '22 parallel to the helix 15, Fig. 8.The point of intersection A will now travel to the right of the hob inits new location A but the hob will still have the roughing series, thefinishing tooth and the burnishing series, as formerly. By the out frombetween the movin the rightend roughing seri'es eading to and connectingwith the said-burnishing series should do so wlthout'forming an intervalor a gap. I consider the practical means of doing this as the mostvaluable part of this invent on.

As diagrammatically shown in Fig. 6, the non-relieved tooth 16 is forcedto take a chip of a thickness e from the workpiece. Let the contactingarea of the tooth be :denoted with F and the cuttin velocity with '0then, the force P with whic the tooth has to be pressed against the workto produce the deformation 0 will be P= bFe 1 where b and c areconstants, the exponent a being of a higher power than one.

The developed heat per second W .will be proportional to the deliveredfrictionwork,

W=dPfv 2 where d is another constant and f the coefiicient of friction.

I observed that thehighly injurious force P will be generated at anamazing rate as soon as the jump e exceeds a'certain ver small limit,maybe less than one thonsandt of an inch. However, for the values of 6less than that the tool will pass through the work freely, in very muchthe same manner as if it were relieved. This action ma be explainedfirst b the structure of the Fquation 1 and second the fact that byexceeding a certain force the oil is suddently squeezed bobbingmachine-thus establishing ametal to metal contact in the said parts.

Therefore, it is imperativev to keep the force P within bounds andthatis most eflectivel done by reducing the jump e and-the area Theangle G included by the first helix 21 and the second helix 15, Fig. 8,may be calculated to produce a jump e not to exceed a predeterminedlimit.

When a hob tooth 16 the lathe tool travelsalong an Archimedean spiral 23having a drop it from tooth to tooth. The said tool also moves paralleltothe axis of the hob thus producing actually a resultant composed of anArchimedean spiral and a straight line, i. e. a conical helix. 7

The clearance angle M at'the top ofthe hob tooth (Fig. 9) may theequation be determined from parts of the Fig. 4, is backed ofl',

, where m is the shortest distance from flute to flute as indicated inFig. 7

Let now K be the normal pressure angle of the hob tooth (Fig. 9) and Mbe the side clearance angle of the said tooth on pitch line, Figs. 7 and8. Then it stands that tan M=tan M tan K (4) The side clearance angle Mwill usually be from 3 to 4 degrees in a coarse pitch high a1=E0 tan M(5) but E0==EA tan G. (6)

now, the thickness of the chip c, Fig. 6 will be equal to where N is thenumber of flutes in the hob.

It is seen now that we can control the thickness of the chip 6 (themaximum which the burnishing tooth will be required to take) by properlyselecting the dimensions fo 1nd in the Equations 3 to 6 inclusive. Inarticular, the whole procedure hinges most upon the selection of thedivergence angle G. I have found out in practical work that the angle Gshould be around 4 to 5 degrees, the angle M about 6 to 7 degrees,thusgiving a bllllllllilShlIlg chip e between .0007 and .001inch t icRegarding the manufacture of these hobs it may be noted that by oneprocess the flutes 19 may be milled first, then the hob is backed offand the flutes 15 are milledafterwards, and by another process theflutes 19 and 15 both may be milled first and the hob may be backed offafterwards. The hob is then hardened and the faces 15 together with thecylindrical lands 14 are ground. The grinding of the lands 14.- may bedone in the same machine in which the worms (of which the hob is acounterpart) are ground, thus insuring a perfect duplication of toothcontours at a low cost.

As shown in Fig. 1 which was traced from a photograph, the cylindricallands 14 are of a more or less irregular shape showing that even whenthe hob as first backed cif was not quite perfect, which is indicated bythe irregularity of the lands, yet such a defect will not impair theability of the hob to produce good work. In this system a single hobtooth may be of such a form that a part of it will be roughing, anotherpart finishing and still another part burnishing without affecting itscorrect working.

In calculating the dimensions of such a hob we first determine the leadof the helical flute 15 such that it will be at right angles to thethread helix aseis customary in all hobs. The angle G is determinedafterwards as was shown in detail and the lead of the auxiliary flute 19is from that calculated.

The principal advantage of this s stem is that now both the roughing andnishing teeth in a hob may be backed off in the same and continuousoperation, thereby producing a series having no jump or interruption atany intermediate point, whereas formerly the roughing teeth had to bebacked off separately by means of two divergent helixes, one for eachside of the roughing tooth Series. This last procedure invariably placedanundue and mostly unforeseen and incalculable excessive loading u onthe first finishing tooth; thusshortening t elife of the hob.

What I claim as my invention is:

1. A rotary thread tool comprising a threaded portion interrupted bylongitudinal flutes forming cuttin teeth in which the flutes are of aconvergin cross section from the entering toward the nishing end and inWhich the cutting teeth are relieved be inning from a line intersectingthe cutting aces at an acute angle at an intermediate point, therebyreducing the entering teeth in thickness and providing the finishingteeth with seri s of non-relieved helicoidal flanks adjacent to thecutting edges and progressive ly increasing in area.

2. A worm hob of the tangential feed t e com rising a threaded portioninterrupte y ap urality of longitudinal flutes forming cutting teeth inwhich the teeth from the entering toward the finishing end are allbacked off with a uniform relief and in an uninterrupted sequence, saidback ofl' beginning from a line intersecting the cutting faces at anacute angle at an intermediate point, thus automatically obtainin adifferentiated series of roughing and finis ing teeth, each toothdiffering from the preceding one either in regards to its thickness atone side of the intermediate point or in regard to the width of thecylindrical lands adj accnt to the cutting edge at the other side of thesaid point.

3. A worm hob of the tangential feed t pe comprising a threaded ortioninterrupted by a plurality of helical flutes forming cutting teeth inwhich the flutes are compounded from two helixes, one of a shorter leadforming the cutting faces and the other of a longer lead forming therear sides of the cuttin teeth and in whichthe teeth are backed 0 alonga helix substantially parallel to the said second helix of the longerlead.

4. A worm hob having a threaded portion and a plurality of compoundflutes, each flute being compounded from two divergent helixes in whichone helix forming the cutting faces is substantially perpendicular tothe hob thread and the second helix forming the rear ends of the cuttingteeth is diverging from the first helix at an angle and is substantiallyparallel to the helix at which the f hob teeth are backed off.

' 5. A worm hob of the tangential feed type comprising a threadedportion interrupted by a plurality of longitudinal flutes formingcutting teeth in which the teeth from 1 the enterin toward the finishingend are backed off fieginning from a line intersecting the cutting facesat an acute angle at an intermediate oint, thus automatically obtaininga di erentiated series of roughing and finishing teeth, each toothdiffering from the preceding one in regard to its thickness at one sideof the intermediate point and in regard to the width of the cylindricallands adjacent to the cutting edge at the other side 2, of the saidpoint and in which the said lands are corrected in form by a passage ofa grinder in a. helical path relative to the hob axis, thus obtaining apartly ground hob of the same degree of accuracy as if it were groundall over.

6. A worm hob having a plurality of cutting teeth backed off from ahelix disposed at an angle relative to the cutting faces'of the saidteeth and a ground over in a helical path concentric with andequidistant from the hob axis, thus producing a series of correctivecylindrical helical surfaces adjacent to the cutting edges andincreasing the cutting accuracy of the hob.

In testimony whereof I afiix my kgnature. I I

NIKOLA TRBOJ E CH.

